+ * @return Pointer to decoded body (might be in original string), or NULL on error
+ *
+ * This does an RFC 822-style parse and honors Content-Transfer-Encoding as
+ * described in <a href="http://tools.ietf.org/html/rfc2045#section-6">RFC 2045
+ * s6</a>. @p callback is called for each header field encountered, in order,
+ * with ASCII characters in the header name forced to lower case.